Conversion of alpha-helices into beta-sheets features in the formation of the scrapie prion proteins.

TLDR

Prions are composed largely of the scrapie prion protein PrPSc, which arises from the cellular prion protein PrPC through a post‑translational process that appears to involve a conformational change rather than a chemical modification. The study aimed to determine whether PrPC converts to PrPSc via a structural transition. Both PrPC and PrPSc were purified by nondenaturing procedures and their secondary structures were characterized by Fourier‑transform infrared and circular dichroism spectroscopy. FTIR and CD analyses revealed that PrPC is predominantly alpha‑helical (42%) with negligible beta‑sheet, whereas PrPSc contains 43% beta‑sheet and 30% alpha‑helix, and the truncated PrP27‑30 shows even higher beta‑sheet (54%) and lower alpha‑helix (21%), indicating that conversion of alpha‑helices into beta‑sheets underlies PrPSc formation and likely drives prion propagation, though a minor chemical modification cannot be ruled out.

Abstract

Prions are composed largely, if not entirely, of prion protein (PrPSc in the case of scrapie). Although the formation of PrPSc from the cellular prion protein (PrPC) is a post-translational process, no candidate chemical modification was identified, suggesting that a conformational change features in PrPSc synthesis. To assess this possibility, we purified both PrPC and PrPSc by using nondenaturing procedures and determined the secondary structure of each. Fourier-transform infrared (FTIR) spectroscopy demonstrated that PrPC has a high alpha-helix content (42%) and no beta-sheet (3%), findings that were confirmed by circular dichroism measurements. In contrast, the beta-sheet content of PrPSc was 43% and the alpha-helix 30% as measured by FTIR. As determined in earlier studies, N-terminally truncated PrPSc derived by limited proteolysis, designated PrP 27-30, has an even higher beta-sheet content (54%) and a lower alpha-helix content (21%). Neither PrPC nor PrPSc formed aggregates detectable by electron microscopy, while PrP 27-30 polymerized into rod-shaped amyloids. While the foregoing findings argue that the conversion of alpha-helices into beta-sheets underlies the formation of PrPSc, we cannot eliminate the possibility that an undetected chemical modification of a small fraction of PrPSc initiates this process. Since PrPSc seems to be the only component of the "infectious" prion particle, it is likely that this conformational transition is a fundamental event in the propagation of prions.